Add patch content regeneration feature in `git go-patch` `rebase`/`apply`

The vendoring patch says:

https://github.com/microsoft/go/blob/f88806da40882a7a451db0922dc30254930c186e/patches/0006-Vendor-crypto-backends.patch#L6

I think we could set up a way to do this automatically when necessary and/or on demand.

The first thing that comes to mind is putting a shell command in the patch file. However, this could be tricky cross-platform and it might be hard to avoid some other environment assumptions, so maybe it would be enough to have a set of built-in logic that handles the cases we care about. Another potential issue is that you might want some flexibility: maybe run gotip mod vendor if go isn't pointing at a recent build of this repo--implementing regen commands in Go would make it easier to add flexibility.

This isn't a big deal with vendoring, because the command is pretty simple and it's actually documented inside the upstream code, too. However, https://github.com/microsoft/go/pull/960 is adding some more code gen, and we might want to apply a regen command on each backend patch: https://github.com/microsoft/go/pull/960#discussion_r1240109730. Running the regen logic is a maintenance burden that we can improve on.

I'm not totally sure yet that this is worth the effort, but I think it's worth tracking. (I thought I'd submitted an issue for it a while back, but can't find one.)

• This could combine with https://github.com/microsoft/go/issues/542 to make upstream sync pretty smart: also attempt to run the regeneration scripts to see if that solves merge conflicts.

Start by reading patches/0006-Vendor-crypto-backends.patch and the git go-patch rebase/apply entry points, then compare the vendoring flow with the regeneration needs described in Microsoft Go PR 960. Done would be a defined, cross-platform way to run the relevant regeneration logic automatically or on demand, with its supported cases and environment assumptions documented.